1. Field of the Invention
The present invention relates to electrography, and more particularly an electrographic imaging or image formation device as well as a developer thickness-controlling blade used in combination with a developing roller in a developing station of the imaging device and a production process for the blade. The developer thickness-controlling blade can effectively control a layer thickness of the developer distributed over a surface of the developing roller and, at the same time, triboelectrically charge the distributed developer, and accordingly it can be advantageously used in electrographic imaging devices using a nonmagnetic one-component type developer. The term "electrography" used herein means that it includes any image formation processes wherein a latent image is first produced on an image-carrying element and it is then developed with a developer to form the corresponding visible image, for example, electrophotography, xerography and the like.
2. Description of the Related Art
In recent years, with development of office automation, electrographic imaging devices such as laser printers have been used as, or in, copiers or copying machines, facsimile devices, output terminal devices for computers and others.
The conventional electrographic imaging device uses the series of process steps which include:
(a) image exposure; PA1 (b) development; PA1 (c) image transfer; PA1 (d) cleaning of the residual toners; and PA1 (e) pre-charging; PA1 providing a press mold comprising at least one pair of a punch and a die, PA1 adjusting a gap between the punch and die to have a size sufficient to give a curved tail part to a tip portion of the blade during the subsequent punching step, and PA1 punching a thin resilient plate of metal in said press mold to thereby form said blade, a tip portion of which has a curved tail part having a smooth surface and a gradually reduced thickness formed as a shear drop, said tail part being able to be pressed against and elastically contacted with a developer-carrying surface of the developing roller in an electrographic imaging process.
and these steps are repeated. In the image exposure step, an image-carrying element such as a photosensitive drum is exposed to image-forming light, i.e., a light image, in an optical unit such as a LED array, to form a charged latent image after it has been sensitized by electrical charging in a preceding pre-charging step. The latent image is formed as a function of a photoconductive discharge of the electrically charged surface of the photosensitive drum. The formed latent image is physically developed using a toner or toning agent as a developer in a developing device. A visible image of the toner is formed on the drum surface as a result of an electrical attraction of fine particles of the toner thereto. The developed image of the toner is then transferred to an image-receiving element such as paper, and the transferred toner image is fixed thereon by fusing. In this image transfer step, some small amount of the toner remains on the surface of the photosensitive drum without being transferred to the paper, and it can adversely affect on the results of the subsequent imaging process if it is not removed from the drum. It is therefore essential in the conventional imaging process to remove the residual toner from the drum in a cleaning step, prior to reusing the drum for the next imaging process. After cleaning thereof, the drum is again sensitized, by electrical charging, in a pre-charging step.
The conventional developers used in the developing device which is also referred hereinafter to as a "developing station" include a one-component type developer essentially consisting of a toner and a two-component type developer essentially consisting of a toner and a carrier. Since it does not contain a carrier which will be deteriorated with time and must be mixed with a toner with the exactly calculated mixing ratio, the one-component type developer can be advantageously used with additional advantage that the constitution of the device used can be simplified.
The one-component type developer, when used in the imaging device, requires essential steps to compulsively electrify the developer and then adhere it onto a surface of the developing roller after application of an electric charge to the roller, because it does not contain a carrier and therefore it cannot adhere to a magnet roller as in the imaging device designed to use a two-component type developer.
Due to the above reason, to assist in electrification of the developer, the toner used in the one-component type developer generally has a relatively high volume resistivity. Further, when the toner used has a volume resistivity in the range of, for example, 10.sup.10 .OMEGA..multidot.cm to 10.sup.13 .OMEGA..multidot.cm or more, since it is essential to compulsively electrify the toner to obtain a predetermined polarity, a triboelectrification element or means has been widely used in combination with a developing roller in the developing station to thereby apply the triboelectrific charge to the toner.
The conventional triboelectrification means include, for example, a blade for adjusting a thickness of the toner on the developing roller to a predetermined and uniform level, to which blade a triboelectrification function was additionally introduced, and a separate electrification means exclusively used for the triboelectrification of the toner. The former means, namely, the double function blade is particularly useful, because it can simultaneously satisfy the requirements for (1) control of the toner thickness and (2) triboelectrification of the toner, and accordingly can simplify the structure of the imaging device including the developing station, in addition to reduction of the production costs of the device. Note that, as will be apparent from the detailed description of this specification, the target of the present invention is to improve the double function blade for use in combination with a developing roller in the imaging device, however, if necessary, the blade of the present invention may be used in the imaging device for a sole purpose of controlling a toner thickness or triboelectrifying the toner.
Hitherto, many types of developer (toner) thickness-controlling blades have been widely used in an electrographic imaging device, and some typical examples of the conventional blades will be described hereinafter with reference to FIGS. 1 to 5 which illustrate only a photosensitive drum 1 which is rotatable in the direction shown by an arrow "A", a developing roller 2 rotatable in the direction "B" and a blade 50 for the purpose of clarifying the function of the blade 50 in the illustrated device.
Referring to FIG. 1, a blade 50 is included in a blade holder 51 and a tip portion thereof can be elastically contacted with a surface of developing roller 2, because a predetermined pushing pressure is applied to the blade 50 from a coil spring 52 in the blade holder 51. The blade 50 is made from a relatively hard resin material or metal, and has a configuration of rectangular plate having a polished tip surface and a thickness of about 2 to 4 mm.
The blade 50 can be constantly contacted with the surface of the roller 2 at a pressure which can be modified depending upon the characteristics of the coil spring 52, however, some problems are generated. For example, due to contact of the roller 2 with the blade 50 under a high pressure and for a long time, creep and thus strain is produced in a surface of the roller 2, and an undesirably increased thickness of the toner is resulted in such strain-generated portion of the roller 2. The unevenness of the toner thickness means that laterally extending stripe-like defects may be produced in the resulting images.
Further, if the blade 50 used has an unexactly fabricated end contacting surface and edge portion, the resulting toner layer tends to have varied layer thickness in the axial direction of the roller 2, and at the same time, show insufficient triboelectrification of the toner.
Furthermore, as a result of the above-mentioned defects, further problems such as unevenness in the density of the resulting images and so-called "fogging" (partial stain) in the background of the resulting images can be generated. Moreover, since friction of the blade 50 against the roller 2 can also act against the rotational direction of the roller 2, the blade 50 can tilt slightly with regard to the blade holder 51 and thus it cannot freely move lengthwise depending upon the movement such as torsional movement of the roller 2. Such insufficient movement of the blade 50 results in the varied toner thickness and partial fogging in conformity with the rotative period of the roller 2, and in the deteriorated quality of the resulting images.
Another prior art blade is illustrated in FIG. 2. The blade 50, as is illustrated, comprises a blade holder 51 having fixed to an end portion thereof a L-shaped blade 50. The blade 50 is produced from a rigid material such as stainless steel, and due to its good elasticity, the corner portion of the blade 50 can be contacted with a surface of the developing roller 2 under a controlled and constant pressure.
However, small cracks or wrinkles can be produced in an edge of L-shaped corner portion during fabrication of the blade 50, and such small defects can adversely affect on the toner passed through a gap between the surface of the roller 2 and the corner portion of the blade 50. Namely, the toner particles are subjected to stress or a grinding action due to the cracks or wrinkles, and the thus finely pulverized. Since the toner particles are deteriorated due to the pulverization thereof, the level of the electrification of the toner particles is lowered with increase in the use time of the toner, and thus "fogging" (reduction of the quality) is caused in the images.
Another prior art blade is illustrated in FIG. 3. A blade 50 is made from an elastic material such as synthetic rubber, and is attached with an adhesive to a blade holder 51. A top end of the blade 50 is contacted with a surface of the developing roller 2 under moderate pressure.
Since it is made from the rubber or similar material, creep can be produced in the blade 50 with repeated use thereof, and thus the pressing power of the blade 5 against the roller 2 can become gradually lower, thereby resulting in reduction of the capability of triboelectrifying the toner which means the gradual formation of deteriorated images. Further, when a silicone rubber or fluoro rubber is used as the blade material in order to improve a release of the toner from the blade, a problem concerning insufficient adhesion of the blade 50 to the holder 51 is induced because of the composition of the rubber, particularly the presence of silicon or fluorine atoms. Further, since the properties such as dimensional stability of the rubber can be widely varied depending upon the environmental conditions such as temperature and humidity, the blade 50 cannot constantly show its desired properties such as durability and adhesion to the holder.
FIG. 4 illustrates one modification of the blade which was explained referring to FIG. 2. In this instance, a blade 50 is made from a rigid material such as stainless steel as in the instance of FIG. 2, however, a configuration of the blade 50 was changed from "L"-shaped cross section to "U"-shaped cross section.
Using the U-shaped blade 50, since it has a rounded corner problems observed with use of the L-shaped blade can be avoided or at least diminished. However, contrary to this advantage, there is a problem that the toner can easily adhere and fix to the blade 50, because a higher contacting pressure is applied to the blade 50 in order to compensate for the difficulty in providing a thin toner layer due to the constitution of the blade, thereby causing fusion of the toner. Fixation of the toner to the blade 50 will produce longitudinal stripe defects or other detects in the images. Further, it is difficult to ensure a constant contact of the blade 50 with the roller 2 under the predetermined pressure, because a spring coefficient of the blade 50 is increased as a function of the increase of the thickness thereof, and the increase of the blade thickness is unavoidable in the production of the blade 50, since if a relatively thin plate is used in the production of the blade 50, a smooth surface cannot be obtained in the resulting blade 50. Furthermore, the production itself of the U-shaped blade 50 from a straight plate is very difficult, and cannot be accomplished without any defect when using a simple machining process.
FIG. 5 illustrates a blade 50 which is constituted from a flat spring, and an end portion of which is fixedly mounted on a blade holder 51. A tip portion of the blade 50 has a round surface produced upon a round edge fabrication, and can be contacted with a developing roller 2 at a predetermined pressure. Since it was produced from a rolled plate having a relatively large thickness of 0.1 to 0.2 mm, the blade 50 suffers from an unevenness of the surface due to its rolled state, and therefore it is deformed when mounted to the blade holder 50. Deformation such as longitudinal waving or corrugation of the blade 50 results in varied thickness of the toner which means that undesirable differences in the density or partial "fogging" can be produced in the resulting images. Further, since its edge is subjected to a drawing process with cutting, the edge cannot be produced with a highly increased machining accuracy and reliability. This also results in "fogging", varied image density and other drawbacks, thereby deteriorating the quality of the images.
As is apparent from the above description, different toner thickness-controlling blades have been proposed for use in an electrographic imaging device in which a nonmagnetic one-component type toner is used as the developer, however, none of them could satisfy the requirements, i.e., constant contact of the blade with the developing roller under the predetermined pressure, control of the thickness of the toner layer at the predetermined level, and uniform electrification of the toner without any deterioration thereof.
Other types of toner thickness-controlling blades have been proposed in Japanese Unexamined Patent Publication (Kokai) Nos. 4-355777 and 6-130801. The blade disclosed in JP-A 4-355777 is directed to reduce a thickness of the toner layer in the dry developing device, and comprises a resilient member 53 and a blade 50 of the rubber material fixed to one end of the resilient member 53 as is illustrated in FIG. 6. The blade 50 has a length (l) of 2 to 15 mm and thickness (t) of 1 mm or more, and a curved edge thereof is contacted with a developing roller 2 under suitable pressure, thereby reducing a layer thickness of toner 4. In this instance, resilience of the resilient member 53 and elasticity of the rubber blade 50 can be effectively combined to obtain the above effects. However, since the triboelectrification of the toner 4 relies upon a silicone rubber or fluoro rubber constituting the blade 50, the toner 4 can be deteriorated due to large friction between the blade 50 and the roller 2 or discontinuous line images can be produced due to momentary stopping of the roller 2 caused because said large friction results in an increase of the load torque on the roller 2.
The blade disclosed in JP-A 6-130801 is directed to inhibit a stress applied to a developer in a developing device, thereby extending a duration of life of the developer. As illustrated in FIG. 7, a blade 31 is disposed substantially perpendicular to a surface of developing roller 2 in such manner that a gap is formed between a tip of the blade 31 and the surface of roller 2. The reference numeral 32 is a sensor for toner concentration. The blade 31 is produced by punching a metal plate, and diagonally cutting and polishing a back half portion of the tip of the resulting blade, while retaining an arch in a front half portion of the same tip. The thus produced blade 31, when disposed over the roller 2, can effectively control piling up of magnetic brushes of carriers over the roller surface, while diminishing the stress on the developer.
Apparently, JP-A 6-130801 has an object to solve a problem of the two-component type developer, i.e., undesirable piling up of magnetic brushes of carriers in said developer, and to solve this problem, it teaches how a tip of blade should be cut and polished to obtain a gap sufficient to control piling up of magnetic brushes.